PREPARATION OF A SILVER CHLOROBROMIDE PHOTOGRAPHIC EMULSION AND USE IN COLOUR NEGATIVE PROCESSES

Abstract: The present invention concerns a process for obtaining a silver halide photographic emulsion which can be used in colour negative photographic processes. This emulsion comprises grains containing a silver chlorobromide core and a peripheral area with a high silver chlorobromide content, and is chemically and spectrally sensitised. This emulsion has improved sensitivity characteristics.

Description: PREPARATION OF A SILVER CHLOROBROMIDE PHOTOGRAPHIC EMULSION AND USE IN COLOUR NEGATIVE PROCESSES The present invention concerns a process for obtaining a silver halide emulsion for colour photography containing chemically and spectrally sensitised silver halide grains and having a structure with a core and a shell (referred to hereinafter as core-shell) . The present invention also concerns the use of such silver halide emulsions in colour negative processes. Although silver halide photographic emulsions are widely known, there is a constant need for photographic products with improved photographic properties, such as greater sensitivity and better granularity. One of the drawbacks of emulsions is that the sensitising dyes used to sensitise the emulsion spectrally, for example cyanine dyes, may cause a loss of intrinsic sensitivity of the emulsion. This loss of sensitivity, or desensitisation due to dyes is recognised as being a problem and is mentioned, for example, in "The Theory of the Photographic Process", 4th Edition, Macmillan Publishing Co. Inc., page 265 et seq. Given the considerable variety of sensitising dyes available, it might be thought that, if a particular dye produces unacceptable desensitisation, a practical solution would consist of choosing another one having substantially the same sensitisation characteristics without desensitisation. However, optimum sensitisation of a silver halide emulsion is generally the result of a complex compromise between several factors, and there are cases where specific dyes are used in spite of their desensitising properties because these dyes have for example the desired adsorption on the grains or absorb light in the desired region of the spectrum. It is also known that increasing the chloride content of the silver halide grains may increase the developability of these grains and enable a fast processing of the emulsion. The emulsions with a high chloride content, this chloride being distributed in one or more phases or areas of the grain, are described, for example, in US patents 4 590 155 and 3 935 014 or in UK patent 1 027 146. However, it is known that chloride emulsions suffer from various deficiencies, such as insufficient stability or reduced sensitivity, compared with bromide or bromoiodide emulsions. There exists a specific need for chloride-containing emulsions which are able to exhibit an optimum sensitivity/grain position without the above-mentioned drawbacks associated with the use of certain sensitising dyes. The present invention provides a means to eliminate the phenomenon of desensitisation generally caused by some sensitising dyes, and enables these dyes to be used without causing the undesirable effects which might have been expected. Accordingly, one object of the present invention is a process for obtaining a spectrally and chemically sensitised silver halide negative emulsion, the said process comprising the steps of (a) forming a silver bromide or bromoiodide core emulsion, (b) forming at the periphery of this core emulsion an outer layer of an emulsion which is (i) substantially free of iodide and (ii) contains from 1 to 50 % molar of chloride based on silver, and (c) effecting the optimal chemical sensitization of said emulsion. wherein, a spectral sensitising dye corresponding to the following formula is adsorbed on the said grains during the step of chemical sensitisation of these grains: where Z 1 and Z 2 represent independently the atoms required to complete a benzoxazole radical, a naphthoxazole radical, a benzothiazole radical, a naphthothiazole radical, a benzimidazole radical or a naphthimidazole radical, in which the aromatic cycles may be substituted by groups such as the alkyl, aryl, aralkyl, alkaryl, halogeno or cyano groups; L 1 and L 2 represent independently a methine group, such as -CH =, C(CH 3 ) =, -C(C 2 H 5 ) = ; R 1 and R 2 represent independently a sulphoalkyl radical, such as β-sulphoethyl, ω-1-sulphobutyl; and n is l or 2 Examples of dyes corresponding to the above formula (I) and appropriated for the purpose of the present invention are: Compared with a control emulsion using the same sensitising dye and having similar characteristics, except that the chloride is replaced by bromide, the emulsion of the invention makes it possible to compensate for the loss of sensitivity resulting from the desensitisation due to the dye and even to increase the sensitivity. According to the present invention, the terms "substantially free" mean that the outer layer of the grain contains only a small quantity of iodide, generally not more than about 0.5 % molar. The process of the invention for producing a chemically and spectrally sensitised silver halide photographic emulsion consists of forming a silver halide core emulsion, forming, at the periphery of this emulsion, an area which is practically free of iodide and contains from 1 to 50% molar chloride and, during the chemical sensitisation stage, adsorbing a sensitising dye as previously defined on the surface of the grains of the emulsion obtained. According to one embodiment, the emulsion comprises a chlorohalide peripheral area or shell so that, the chloride is distributed on the periphery of the grain, and in this peripheral area the chloride content may vary abruptly or gradually, forming areas with a high or low chloride content, within the ranges previously mentioned. As indicated, the chlorobromide shell of an emulsion according to the invention comprises from 1 to 50% molar chloride ; the shell preferably contains from 1 to 30% molar chloride and more preferably from 2 to 20% molar chloride based on total silver in the grain. According to a preferred embodiment, the emulsion of the present invention comprises silver halide grains containing a core and one or more shells, the composition of the halides being different in the core and the shell or shells. The peripheral area containing the chloride preferably represents a thickness greater than 100 A and less than 60% of the radius of the grain. Advantageously, the thickness of the peripheral area containing the chloride represents from 10 to 50% of the radius of the grain. The silver halide grains have the crystalline structures generally used in silver halide photography. They have crystalline structures of the cubic halite type with isomorphic aligned faces. The core of the grains may consist of silver bromide, silver bromoiodide, silver chloroiodide or mixtures of these. When the grains contain iodide in the core, the maximum possible quantity of iodide is the quantity which can be accepted by the cubic crystal lattice. In a silver bromide cubic crystal lattice, up to about 40% molar iodide can be incorporated, depending on the precipitation temperature. However, when iodide is used in the core, the grains preferably contain less than about 15% molar iodide. In conventional photographic applications, the iodide concentrations are generally below about 20% molar. The distribution of the iodide may be homogeneous (for example, resulting from precipitation by a continuous jet of iodide) or discrete, the structures resulting from a sudden addition of iodide during precipitation. The emulsions may comprise large, medium or fine silver halide grains, predominantly delimited by crystal planes [100] or [111], and they may be regular or irregular in shape, for example cubic or octahedral. The emulsions prepared according to the process of the invention preferably consist of cubic, octahedral or cubo- octahedral grains, comprising a bromide or bromoiodide core and an outer peripheral area with a high chloride content, as mentioned above. The emulsions forming the core of the grains may be prepared using various techniques, for example single-jet, double-jet or accelerated-flow precipitation techniques as described by Trivelli and Smith, The Photographic Journal. Vol. LXXIX, May 1939, pp. 330-338, by T H James, The Theory of the Photographic Process. 4th Edition, Macmillan, 1977, Chapter 3, by Niertz et al in US patent 2 222 264, by Wilgus in the German patent application No 2 107 118, by Lewis in UK patents 1 335 925, 1 430 465 and 1 469 480, by Irie et al in US patent 3 650 757, by Morgan in US patent 3 917 485, by Musliner in US patent 3 979 213, in Research Disclosure. Article 17643, Vol. 175, December 1978, and Research Disclosure. Article 22434, Vol. 225, January 1983. Research Disclosure and its predecessor, Product Licensing Index, are publications of Industrial Opportunities Ltd, Homewell, Havant, Hampshire, P09 1EF, United Kingdom. Modifying compounds may be present during the precipitation of the grains. Such compounds may be present in the reaction vessel initially or they may be added at the same time as one or more salts, in accordance with conventional operating methods. The modifying compounds, such as the middle chalcogens (namely sulphur, selenium and tellurium) , gold and the noble Group VIII metals (for example iridium) , may be present during the precipitation of the halides, as described in Research Disclosure, Vol. 134, June 1975, Article 13452. An important characteristic of the invention is that the chloride is coprecipitated with another silver halide, so as to form a mixed phase of silver chlorohalide in the outer peripheral area of the grain. According to a preferred embodiment, the process of the invention consists of precipitating a bromide or bromoiodide core emulsion, and then coprecipitating a chlorobromide emulsion on the core emulsion. Alternatively the bromide or bromoiodide core emulsion can first be shelled with a bromide emulsion which will itself be shelled by coprecipitating a chlorobromide emulsion. The parameters for these precipitation stages, such as the pAg, the flow rates, the flow profiles, the salt concentrations and the number of moles precipitated as a function of time and temperature, largely depend on the type of emulsion in question and on the desired results, and will consequently be determined in each case by a person skilled in the art. The emulsions are sized using a grain volume analyser and the determination is based on the principle of direct electrolytic reduction. Such an apparatus is described by A Holland and A Feiner an in J. Applied Photo. Eng. 8., 165 (1982) . Such an apparatus makes it possible to obtain a volume distribution of the size of the grains from which a mean distribution volume (V) can be calculated, and then an equivalent spherical diameter (ESD) and a standard deviation (SIGMA) , Vi being the volume of a given grain and N being the number of grains counted. 3 X V ESD = x - in micrometres 3 x π (Vi - V) SIGMA = Σ 1 N The coefficient of variation (COV) is defined as representing 100 times the ratio of the volume standard deviation to the mean volume of the grain population SIGMA COV = 100 X V These emulsions are preferably monodisperse, that is to say they have a narrow size distribution characterised, for example, by a coefficient of variation of less than 20% and preferably less than 10%. The emulsions obtained according to the invention are intended for negative working black and white or colour processes. The colour materials generally comprise a support covered with at least two layers of silver halide emulsion with which are associated one or more dye-forming couplers. Colour reversal techniques are well known. These emulsions can be chemically sensitised by any conventional technique or using any conventional sensitiser, such as those indicated in Research Disclosure No 308, December 1989, paragraph III. These sensitisers comprise, for example, active gelatin, as described by T H James, The Theory of the Photographic Process. 4th Edition, Macmillan; 1977, pages 67-76, or sulphur, selenium, tellurium, gold, platinum, palladium, iridiu , osmium. rhodium, rhenium or phosphorus sensitisers or combinations of these sensitisers, at pAg values between 5 and 10, pH levels between 5 and 8 and temperatures between 30° and 80°C, as described in Research Disclosure, Vol 120, April 1974, Article 12008, Research Disclosure, Vol 134, June 1975, Article 13452, by Sheppard et al in US patent 1 623 499, by Matthies et al in US patent 1 673 522, by Waller et al in US patent 2 339 083, by Da schroder et al in US patent 2 642 361, by McVeigh in US patent 3 297 447, by Dunn in US patent 3 297 446, by McBride in UK patent l 315 755, by Berry et al in US patent 3 772 031, by Gilman et al in US patent 3 761 267, by Ohi et al in US patent 3 857 711, by Klinger et al in US patent 3 565 633, by Oftedahl in US patents 4 901 714 and 3 904 415 and by Simons in UK patent 1 396 696; the chemical sensitisation may optionally be effected in the presence of thiocyanates, preferably at concentrations between 2 x 10~ 3 and 2% molar with respect to the total silver content, as described by Damschroder in US patent 2 642 361; sulphur-containing compounds of the type described in the US patents 2 521 926 of Lowe et al, 3 021 215 of Williams et al and 4 054 457 of Bigelow. Specifically, it is considered that chemical sensitisation can be effected in the presence of compounds which modify chemical sensitisation, that is to say compounds " known to eliminate fogging and increase sensitivity when they are present during chemical sensitisation, such as the azaindenes, azapyridazines, azapyrimidines and salts of benzothiazolium, and sensitisers comprising one or more heterocyclic rings. Examples of finishing modifiers are described in the US patents 2 131 038 of Brooker et al, 3 411 914 of Dostes, 3 554 757 of Kuwabara et al, 3 565 631 of Oguchi et al and 3 901 714 of Oftedahl, in the Canadian patent 778 723 of Walworth and in Duffin. Photographic Emulsion Chemistry. Focal Press (1966), New York, pages 138-143. In addition, the emulsions may be sensitised by reduction - for example, with hydrogen, as described by Janusonis in US patent 3 891 446 and by Babcock et al in US patent 3 984 249, by a process using a low pAg (for example below 5) and/or a high pH (for example above 8) or by using reducing agents, such as stannous chloride, thiourea dioxide, polyamines and amine boranes, as described by Allen et al in US patent 2 983 609, Oftedahl et al in Research Disclosure, Vol. 136, August 1975, Article 13654, by Lowe et al in US patents 2 518 698 and 2 739 060, by Roberts et al in US patents 2 743 182 and 2 734 183, by Chambers et al in US patent 3 026 203 and by Bigelow et al in US patent 3 361 564. The superficial chemical sensitisation, including sensitisation below the surface, described by Morgan in US patent 3 917 485 and by Becker in US patent 3 966 476, is specifically considered. Preferably the outer peripheral area containing the chloride or chlorobromide is sensitised using gold and/or sulphur. In addition to being sensitised chemically, the silver halide emulsions of the present invention are also sensitised spectrally by means of spectral sensitising dyes of the class defined above. It is considered specifically that spectral sensitising dyes can be used which have maximum absorption levels in the blue and minus blue portions, ie green and red in the visible spectrum. In addition, in specialised applications, spectral sensitising dyes which improve the spectral response beyond the visible spectrum can be used. One or more spectral sensitising dyes can be used. Dyes are known which have maximum sensitisation at various wavelengths in the visible spectrum and a great variety of spectral sensitivity curve forms. The choice and the relative proportions of the dyes depend on the region of the spectrum where it is desired to obtain the sensitivity and on the desired spectral sensitivity curve form. A mixture of sensitising dyes can be used with partially overlapping absorption spectra; such a mixture can give a spectral sensitivity which, at each wavelength in the overlap, is at least equal to and sometimes greater than the sum of the individual sensitivities of the individual dyes. Mixtures of the dyes specified above can also be used with other conventional sensitising dyes. A characteristic of the present invention is that the sensitising dye is added during the chemical sensitisation of the emulsion, as indicated in the following examples. The indicated silver potentials are measured by means of a saturated calomel electrode, unless otherwise indicated. EXAMPLE 1 - (Reference) 4.156 litres of distiled water and 57.8 g/1 of deionised phthalylated gelatin are introduced into a 20 litre precipitation reactor and heated to 60°C. An anti-foaming agent and a ripening agent (thioether) are added before adjusting the pAg to -9.1 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of microcrystals of AgBr is effected by introducing 0.035 moles of AgN0 3 and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of +9.1 at a temperature of 60°C. This nucleation is followed by a hold period of 120 seconds. This hold period is followed by a period of initial growth for 33.3 minutes, in which 2.0 M/l of AgNθ 3 , 1.82 M/l of NaBr and 0.18 M/l of KI are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and the pAg at the same values as before. 3.35 moles of silver halide are precipitated in this initial growth stage. The precipitation is ended by a final growth segment lasting 29 minutes in which 2.0 M/l of AgN0 3 and 2.0 M/l of NaBr are introduced by double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. 6.66 moles of silver halides are precipitated during this second growth stage. The emulsion is then washed using a conventional flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 1.135 micrometres and a total iodide content of 3% molar. This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is carried out for 20 minutes at 70°C using: 200 mg of sodium thiocyanate per mole of Ag 3.08 mg of sulphur in the form of Na 2 S 2 θ 3 ,5H 2 θ per mole of Ag 2.05 mg of gold in the form of potassium tetrachloroaurate per mole of Ag. Simultaneously with the chemical sensitisation stage, a mixture of 125.7 mg of sensitising dye (formula A) and 26.3 mg of sensitising dye (formula B) per mole of Ag is added for green sensitisation. Dye A has the formula : Dye B has the formula : An octahedral emulsion with a core and shell of sensitised bromide is obtained, having an equivalent spherical diameter (used as the mean dimension of the grains) of 1.135 μm, and a volume coefficient of variation of 7.1%. The total iodide content corresponds to 3% molar and the iodide content of the core corresponds to 9% molar. This bromide-shell emulsion (emulsion El) is used as a control. EXAMPLE 2 - (Invention) 4.156 litres of distilled water and 57.8 g/1 of deionised phthalated gelatin are introduced into a 20 litre precipitation vessel and heated to 60°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 9.10 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of icrocrystals of AgBr is effected by introducing 0.035 moles of AgN0 3 and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as obtain a pAg of 9.10 at a temperature of 60°C. This nucleation is followed by a hold period of 120 seconds. This hold period is followed a period of initial growth lasting 33.3 minutes, during which 2.0 M/l of AgN0 3 , 1.82 M/l of NaBr and 0.18 M/l of KI are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and the pAg at the same values as before. A total of 3.35 moles of silver halides are precipitated during this initial growth stage. Following this initial growth stage the pAg is modified to a value of 7.11 using a solution of 2.0 M/l of AgN0 3 introduced over a period of 156 seconds. This shift in the pAg is followed by a hold period of 60 seconds. The hold period is followed by a final growth segment. The final growth segment is precipitated over a period of 39 minutes, during which 2.0 M/l of AgN0 3 , 1.70 M/l of. NaBr and 0.30 M/l of NaCl are introduced by double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. A total of 6.66 moles of silver halides are precipitated during this second growth stage. The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 1.107 μm and a total iodide content of 3% molar. This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is carried out for 20 minutes at 70°C using: 200 mg of sodium thiocyanate per mole of Ag 2.0 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 1.33 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, and then simultaneously with the chemical sensitisation, a mixture of 166.0 mg of sensitising dye (formula A) per mole of Ag and 48.0 mg of sensitising dye (formula B) per mole of Ag is added for green sensitisation. The emulsion obtained (E2) is a core and shell cubo- octahedral emulsion containing 15% molar chloride in the shell in which silver chloride and silver bromide have been coprecipitated, having an equivalent spherical diameter of 1.107 μm and a volume coefficient of variation of 6.2%. EXAMPLE 3 - (Control) 4.156 litres of distiled water and 57.8 g/1 of deionised phthalated gelatin are introduced into a 20 litre precipitation reactor and heated to 60°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 9.10 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of AgBr microcrystals is effected by introducing 0.035 moles of AgN0 3 and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 9.10 at a temperature of 60°C. This nucleation is followed by a hold period of 120 seconds. This hold period is followed by a period of initial growth lasting 33.3 minutes, during which 2.0 M/l of AgN0 3 and 2.0 M/l of NaBr are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBrI while maintaining the temperature and pAg at the same values as before. A total of 3.36 moles of silver halides are precipitated during this initial growth stage. For the final growth segment, 2.0 M/l of AgN0 3 and 2.0 M/l of NaBr are introduced over a period of 29 minutes by double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. 6.66 moles of silver halides are precipitated during this second growth stage. The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains having an equivalent spherical diameter of 1.05 micrometres. This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected for 20 minutes at 70°C using: 250 mg of sodium thiocyanate per mole of Ag 3.5 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 2.92 g of gold in the form of potassium tetrachloroaurate per mole of Ag. A mixture of 156.5 mg of sensitising dye (formula A) per mole of Ag and 45.2 mg of sensitising dye (formula B) per mole of Ag is added for green sensitisation during the sensitisation stage. A sensitised octahedral emulsion is obtained with an equivalent spherical diameter (used as the mean grain dimension) of 1.05 μm and a volume coefficient of variation of 6.8%. This bromide emulsion (Emulsion E3) is used as a reference. EXAMPLE 4 - (Control) 4.156 litres of distiled water and 57.8 g/1 of deionised phthalated gelatin are introduced into a 20 litre precipitation reactor and heated to 60°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 9.10 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of microcrystals of AgBr is effected by introducing 0.035 moles of AgN0 3 and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 9.10 at a temperature of 60°C. This nucleation is followed by a hold period of 120 seconds. This hold period of inactivity is followed by a period of initial growth lasting 41.2 minutes, during which 2.0 M/l of AgN0 3 and 2.0 M/l of NaBr are introduced, by accelerated double-jet precipitation, so as to produce a mixed phase of AgBr while maintaining the temperature and pAg at the same values as before. A total of 3.50 moles of silver halides are precipitated in this initial growth stage. This initial growth stage is followed by a shift pAg to a value of 7.11 using a solution of 2.0 M/l of AgN0 3 introduced over a period of 160 seconds. This shift pAg is followed by a final growth segment. For the final growth segment, 2.0 M/l of AgN0 3 and 2.0 M/l of NaBr are introduced over a period of 36 minutes by accelerated double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. A total of 5.42 moles of silver halides are precipitated in the course of this second growth stage. The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains with an equivalent spherical diameter of 1.00 micrometre. This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected for 20 minutes at 70°C using: 250 mg of sodium thiocyanate per mole of Ag 2.50 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 2.08 mg of gold in the form of potassium tetrachloroaurate per mole of Ag. A mixture of 172.5 mg of sensitising dye (formula A) per mole of Ag and 49.8 mg of sensitising dye (formula B) per mole of Ag is added for green sensitisation, during the chemical sensitisation stage. The emulsion obtained is a cubo-octahedral emulsion having an equivalent spherical diameter (used as the mean grain dimension) of 1.00 μm, and a volume coefficient of variation of 7.3%. This bromide emulsion (Emulsion E4) is used as a reference. EXAMPLE 5 - (Invention) 4.156 litres of distiled water and 57.8 g/1 of deionised phthalated gelatin are introduced into a 20 litre precipitation reactor and heated to 60°C. An anti-foaming agent and a thioether ripening agent are added before adjusting the pAg to 9.10 using a solution of NaBr, and the pH is adjusted to 5.1. The nucleation of a stable population of microcrystals of AgBr is effect by introducing 0.035 moles of AgN0 and NaBr by double-jet precipitation over a period of 70 seconds, using 0.5 M/l of reagents. An excess of bromide is maintained so as to obtain a pAg of 9.10 at a temperature of 60°C. This nucleation is followed by a hold period of 120 seconds. This hold period of inactivity is followed by a period of initial growth lasting 40.1 minutes, during which 2.0 M/l of AgN0 3 and 2 M/l of NaBr are introduced, by accelerated double- jet precipitation, so as to produce a mixed phase of AgBr while maintaining the temperature and the pAg at the same values as before. A total of 3.38 moles of silver halides are precipitated in this initial growth stage. After this first initial growth, the pAg is adjusted to a value of 7.11 using a solution of 2.0 M/l of AgN0 3 introduced over a period of 150 seconds. This shift pAg is followed by a final growth segment. For the final growth segment, 2.0 M/l of NaBr, 1.7 M/l of NaBr and 0.3 M/l of NaCl are introduced by accelerated double-jet precipitation, while maintaining the temperature and the pAg at the same values as before. A total of 6.6 moles of silver halides are precipitated during this second growth stage. The emulsion is then washed by a conventional flocculation process. The final emulsion consists of octahedral grains with an equivalent spherical diameter of 1.03 micrometres and a total chloride content of 9% molar. This emulsion is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected for 20 minutes at 70°C using: 35 mg of sodium thiocyanate per mole of Ag 1.8 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 1.6 mg of gold in the form of potassium tetrachloroaurate per mole of Ag. A mixture of 182.5 mg of sensitising dye (formula A) per mole of Ag and of 52.5 mg of sensitising dye (formula B) per mole of Ag is added for green sensitisation, and then 40 mg of acetamidophenylmercaptotetrazole per mole of Ag is added. The emulsion obtained (E5) is an octahedral core/shell emulsion containing 9% molar chloride in the part of the shell where the silver chloride and silver bromide have been coprecipitated; this emulsion has an ESD of 1.03 μm and a volume coefficient of variation of 6.9%. EXAMPLE 6 (Control) The (reference) bromide emulsion precipitated as described in Example 4 is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 2.50 mg of sodium thiocyanate per mole of Ag 2.50 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 2.08 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 262.5 mg of sensitising dye C is added per mole of Ag for blue sensitisation. The emulsion obtained (E6) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.00 μm and a volume coefficient of variation of 7.3%. Dye C has the formula: « . s s—.'\ 4- \ / \ 1 fi I ( I I / \ -ϊ CL—• • N N / I l+ι I I 1 1 - 1 -—-.-SQ3H .-.-.-502-0 EXAMPLE 7 (Invention) The bromochloride core/shell emulsion (invention) precipitated as described in Example 5 is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 35 mg of sodium thiocyanate per mole of Ag 1.8 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 1.6 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 274.5 mg of sensitising dye C per mole of Ag is added according to the method during the chemical sensitisation, for blue sensitisation, and then 40 mg of acetamido ercaptophenyltetrazole per mole of Ag is added. The emulsion obtained (E12) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.03 μm and a volume coefficient of variation of 6.9%. EXAMPLE 8 (Control) The (reference) bromide emulsion precipitated as described in Example 4 is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 2.50 mg of sodium thiocyanate per mole of Ag 2.50 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 2.08 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 226.3 mg of sensitising dye (formula D) per mole of Ag is added according to the method during the chemical sensitisation, for blue sensitisation. The emulsion obtained (E13) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which the silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.00 μm and a volume coefficient of variation of 7.3%. Dye D has the formula: The bromochloride core/shell emulsion (invention) precipitated as described in Example ll is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 35 g of sodium thiocyanate per mole of Ag 1.8 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 1.6 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 236.6 mg of sensitising dye D per mole of Ag is added according to the method during the chemical sensitisation, for green sensitisation, and then 40 mg of acetamidomercaptophenyltetrazole per mole of Ag is added. The emulsion obtained (E9) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which the silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.03 μm and a volume coefficient of variation of 6.9%. EXAMPLE 10 (Control) The (reference) bromide emulsion precipitated as described in Example 4 is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 2.50 mg of sodium thiocyanate per mole of Ag 2.50 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 2.08 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 213.6 mg of sensitising dye (formula E) per mole of Ag is added according to the method during the chemical sensitisation, for red sensitisation. The emulsion obtained (ElO) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which the silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.00 μm and a volume coefficient of variation of 7.3%. Dye E has the formula: • s s « ✓ \ κ \ E T / \ • « \ I / m m ( , , = .. _.. __:—, t , Cc — • • • / \ . -— C L • v / \ / / \ *f I 1 + 1 • 1 ( I • • i i • . < ( SO JH • • . I - 1 0=S 0 I EXAMPLE 11 (Invention) 0 The bromochloride core/shell emulsion (invention) precipitated as described in Example 5 is sensitised chemically and spectrally at a point determined as being the optimum position. The chemical sensitisation is effected over a period of 20 minutes at 70°C, using: 35 mg of sodium thiocyanate per mole of Ag 1.8 mg of sulphur in the form of sodium thiosulphate pentahydrate per mole of Ag 1.6 mg of gold in the form of potassium tetrachloroaurate per mole of Ag, 272.9 mg of sensitising dye E per mole of Ag is added according to the method during the chemical sensitisation, for red sensitisation, and then 40 mg of acetamidomercaptophenyltetrazole per mole of Ag is added. The emulsion obtained (Ell) is a cubo-octahedral core/shell emulsion containing 15% molar chloride in the shell, in which the silver chloride and silver bromide have been coprecipitated, and having an equivalent spherical diameter of 1.03 μm and a volume coefficient of variation of 6.9%. EXAMPLE 12 All the above emulsions are applied in a proportion of 8.07 mg/dm 2 of silver, 23.6 mg/dm 2 of gelatin and 16.15 mg/dm 2 of a magenta coupler (compound F) on a triacetate support, and are exposed for 1/100th of a second to a radiation of 5500°K through a Wratten R #9 filter and a neutral density filter. Each sample is processed using a Kodak C-41 R chro ogenic negative process.. The results are set out in Table I. The parameters used in Table I are defined as follows: - toe sensitivity : measured at a density Dl greater than the minimum density for which a variation in sensitivity of 0.60 log E is observed when going from Dl to D2 = 3 x Dl, the sensitivity index (SI) being determined by SI = 100 x (1 - log E D1 ) , - midscale contrast : maximum absolute value of the first derivative of the curve D = f(log E) , - toe contrast : measured between two points of density Dl = D in + 0.1 and D2 = Dmin + 0.4, the value being determined by the following equation: toe contrast at the foot = 0.3/ (log E^ - log E2) - granularity at D — 1 : measured at a density of 1 using a microdensitometer with a 48 μm aperture. These results show that the bromochloride shell emulsion (invention) has a greater sensitivity than the bromide shell emulsion, while barely affecting contrast and granularity. Compound F has the formula: 0 * \ / \ ε t / \ CR \ N H 1 + 1 « ( • « ' f I « 0 « I « - •-S03H I o Emulsion sample Toe Relative sensitivity

Claims: CLAIMS 1. Process for preparing a chemically and spectrally sensitized silver halide photographic emulsion, said process comprising the following steps: (a) forming a silver, a bromide or bromoiodide core emulsion, (b) forming, at the periphery of this core emulsion, and emulsion which is (i) substantially free of iodide and (ii) contains from l to 50 % molar of chloride. (c) effecting the chemical sensitisation, and wherein, during the step (c) , at least one spectral sensitising dye corresponding to the following formula is added: / X Z 1 C - (L 1 = L 2 ) n _ 1 -CH = C 2 -N+ N N-' R 1 R 2 where Z 1 and Z 2 represent independently the atoms required to complete a benzoxazole, naphthoxazole, benzothiazole, naphthothiazole, benzimidazole or naphthi idazole radical, in which the aromatic cycles may be substituted by groups such as the alkyl, aryl, aralkyl, alkaryl, halogeno or cyano groups; L 1 and L 2 represent independently a methine group, such as -CH =, C(CH 3 ) =, -C(C 2 H 5 ) = ; R 1 and R 2 represent independently a sulphoalkyl radical, such as β-sulphoethyl, ω-1-sulphobutyl; and n is 1 or 2. 2. The process of Claim 1, wherein the emulsion grain core contains up to 40% molar iodide based on total silver present in the core. 3. The process of Claim 1, wherein the core-shell grains contain from 1 to 30% molar chloride based on total silver present in the grain. 4. The process of Claim 3, wherein the grains contain from 2 to 20% molar chloride. 5. The process of Claim 1, wherein characterised in that a core emulsion, a first silver bromide shell emulsion and a second outer shell emulsion comprising silver chloride are successively precipitated. 6. The process of Claim 5, characterised in that the shell emulsion containing silver chloride receives an optimum chemical sensitisation.

Inventors: FRIOUR GERARD AMEDE DESIRE, POURCHET SYLVIE MARTINE DANIEL, FRIOUR, GERARD, AMEDE, DESIRE, POURCHET, SYLVIE, MARTINE, DANIELE
Applicants: EASTMAN KODAK CO, FRIOUR GERARD AMEDE DESIRE, KODAK PATHE, POURCHET SYLVIE MARTINE DANIEL, EASTMAN KODAK COMPANY, FRIOUR, GERARD, AMEDE, DESIRE, KODAK-PATHE, POURCHET, SYLVIE, MARTINE, DANIELE
Application year: 1994
Main classification: G03C1
IPC codes: G03C1, G03C7